1. Field of the Invention
This invention relates generally to optical disk reproducing apparatus and, in particular, is directed to an optical disk player using a rewritable disk.
2. Description of the Prior Art
In an optical disk recording and reproducing apparatus of a so-called DRAW system using a writable disk, when a laser light amount per unit area irradiated on the disk exceeds a predetermined value, the writing (or recording) of data is carried out on the disk.
As the disk drive system for recording and/or reproducing data, there are known a CAV (constant angular velocity) drive system in which recording and/or reproducing data on and/or from a disk is carried out under the condition that the revolution number of the disk is kept constant and a CLV (constant linear velocity) drive system in which recording and/or reproducing data on and/or from a disk is carried out under the condition that the linear velocity of the disk is kept constant.
By the way, in the optical disc player of such DRAW system, upon reproducing data, a laser power must be controlled so as not to exceed a light amount by which data can be written on the disk. For example, in an example of the optical disk player of DRAW system capable of employing both the CAV and CLV drive systems to drive a disk with a diameter of 20 cm, in the CAV drive mode, the revolution number of the disk is constant as 900 revolutions/minute (r.p.m.), while in the CLV drive mode, in order that the linear velocity is made constant, the disk is rotated at 900 r.p.m. at its inner periphery and as an optical pick up device or head is moved toward the outer periphery of the disk, the revolution number of the disk is lowered and the disk is rotated at 525 r.p.m. in its outer periphery.
As described above, when the optical disk player is driven in the CAV drive mode, since the revolution number of the disk is constant, the linear velocity is different between the inner periphery and the outer periphery of the disk, or the linear velocity is slow as about 5.2 m/sec at the inner periphery of the disk and becomes higher as the outer periphery of the disk is approached and is about 8.7 m/sec at the outer periphery. Accordingly, in order that the laser power upon writing is made the constant laser power per unit time, the laser power upon writing is controlled such that the laser power becomes about 14 mW at the inner periphery of the disk, while it is increased gradually as the outer periphery of the disk is approached and it becomes about 17 mW at the outer periphery of the disk. Thus, the writing is carried out always at optimum power.
Upon reading (or reproducing), since the reading of the disk is always carried out by a laser power at which value no writing is performed on the disk at its inner periphery where the linear velocity is slow, there is no fear that the rewriting on the disk will be carried out even at the outer periphery of the disk where the linear velocity is high. In this example, when the linear velocity at the inner periphery of the disk is about 5.2 m/sec the laser power upon reading is selected to be about 1.3 mW.
On the other hand, when the optical disk player is driven in the CLV drive mode, since the linear velocity is fixed to be about 5.2 m/sec, upon writing, the laser power is about 14 mW, while upon reading, in order to remove a fear of the rewriting and to improve an S/N (signal-to-noise) ratio, a laser power is desired to be as large as possible and in this example, fixed to be about 1.3 mW.
However, upon reproducing in the CLV drive mode, when the track to be reproduced is changed, if the optical head is jumped from, for example, the outer peripheral track to the inner peripheral track, the optical head can be moved instantly but it takes several seconds to change the revolution number of the disk to the normal revolution number due to the moment of inertia of the disk and so on. As a result, in the reading mode of the inner peripheral track, the revolution number, the linear velocity and the laser power must respectively be 900 r.p.m., 5.2 m/sec and 1.3 mW but will initially be about 525 r.p.m., about 3.0 m/sec and 1.3 mW, respectively, because the disc revolutions cannot change instantaneously. Thus, there will occur such a fear that the laser beam amount will exceed the light amount capable of the writing and hence a useless data will be rewritten into the disk.
To solve this problem, it may be considered to inhibit the emission of the laser beam until the revolution number of the innermost periphery of the disk is locked to a predetermined revolution number. In this case, however, the appointed address of the innermost periphery of the disk can not be accessed and the just tracking state can not be established. As a result, it is necessary to always excite the laser.